Process of separating inorganic salts from solutions



Patented m, 1938 UNITED STATES,

,PAHTIENT OFFICE rnocEssoF snrm'rma INORGANIC SALTS raom SOLUTIONS George'W. Smith, Pittsburgh, Pa... assignor to Hall Laboratories, Inc Pittsburgh, 2a., a corporatlon oflennsyl No Drawing. Application March 12, 1936, Serial No. 88,445

1'; Claims. .(01. 23-50) This invention relates-to aprocess oi. separating inorganic salts from solutions and provides means whereby impurities in the solution, such as alkaline earth metal salts and salts of divalent 5 heavy metals, foraexampleferrous iron and cupric copper, are maintained in solution during precipitation of the inorganic salt from solution. The present invention is applicable to the crystallization of common salt from solutions contalning this salt and, in addition, calcium salts as impurities. Virtually all commercial sodium chloride brines contain quantities of, calcium sulphate. ,These brines are evaporated either in open vessels or under vacuum. As the brine solution becomes saturated. with sodium chloride, crystals of sodium chloride are deposited from solution in the bottom of the pan or crystallizer. At the same time, the solution becomes saturated with calcium salts, principally calcium sulphate.

A part of this calcium sulphate precipitates under certain conditions as scale upon the heating surfaces and a part of it precipitates as calcium sulphate sludge along' with 'the sodium chloride,-

, thereby contaminating the sodium chloride.

2 I have found that the calcium sulphate or other calcium salt. present'in the solution of sodium chloride as an impurity may be prevented from crystaliizlng out of solution with .the sodium chloride upon heating the solution, if an alkalimetal 'hexametaphosphate is added to the solution. The preferred alkali-metal hexametaphos- .phate is sodium hexametaphosphate, sometimes called Grahams salt". The alkali-metal hexametaphosphate has the property of formlng with calcium and magnesium and other divalent heavy metals, soluble very slightly ionized compounds of these metals. In other words, the alkali metal hexametaphosphate has the property of sequestering'the calcium, magnesium and other metals in a axlluble and but slightly-ionized condition so that ese metals do. not precipitate along with the sodium chloride as the sodium chloride crystallizes fromsolution upon heating;

The quantities of sodium hexametaphosphate.

5 for example, which should be used ior preventing precipitation of the calcium sulphate will vary according to the pH value of the solution and according to the solubility of the substance whose precipitation is :to be avoided. If the sodium chloride solution has a pH value of 7, precipitation of calcium sulphate may be prevented by adding to the solution sodium hexametaphosphate in an amount about eight times by weight of the quantity of calcium which is present in the' 6:; solution. The quantities of sodium hexametaphosphate will increase with increasing pH values of the solution and decreasingsolubility of the substance whose precipitation istobe avoided. -Thus, where trisodium phosphate is being crystallized from a brine containing calcium as 5 an impurity, the pH value of-the brine will be high. Furthermore, calcium phosphate is quite insoluble. In this case the amount of sodium hexametaphosphate which should be .used is approximately fifteen times the weight of the cal- 10 cium which is present in *the solution. As another example,gthe invention is useful in carrying out the precipitation of ammonium sulphate from ammonium sulphate brine to which gaseous ammonia and suIphu'ric acid are 15- continuously added. Calcium and magnesium from the water used in making the brine, and any calcium, magnesium, ferrous iron or cupric copper or. other divalent alkaline earth or heavy metal present in the sulphuric acidas impuri- 2o ties, may be held in solution'during the precipi-,

tatlon process by addition to the solutionbf suitable quantities of an alkali-metal hexametaphosphate. Calcium and magnesium find their.

'ployed which will cause conversion of the hexametaphosphate into the orthophos'phate. For example, in the'ca'se of precipitation conducted in baths having atemperature of 200?.0, the cona version would occur very rapidly and the present 40 process would not. be applicable. At lower tem- I peratures, such as 100 C., the sodium hexametaphosphate will havean effective life of two or three hours and the process would, therefore, be'

feasible at atmospheric boilingtemperatures,

provided such temperatures were not maintained for too long a time- Where vacuum is employed in carrying out. the precipitation, the temperature may be as low as to 60 C., and at'these temperatures the hexametaphosphate has an in- 5 definite" life and may be used in processes in which such temperatures are employed for 24 hours or m 'e'. l While I p fer to use sodium' hexametaphosphate, other alkali-metal hexametaphosphates Patent No. 2,168,783.

lithium or ammonium hexametaphosphates.

The invention is applicable to the separation of inorganic salts from solutions containing as imorganic salt.

4. Aprocess of separating an alkali-metal or CERTIFICATE It is hereby certified that. error may be used; such, for example, as potassium,

ammonium salt from a solution containing the same and in addition an impurity oi the group consisting of alkaline earth metal salts and salts of divalentheavy metals, which comprises add- ,GEORGE w. sm'rn.

F CORRECTION I r y 5', 195a. GEORGE'W. s im.

purities any of the alkaline earth or divalent ing an alkali-metal hexametaphosphate to the heavy metals. For example, sodium hexa'metasolution to hold the impurities in solution, and phosphate holds in solution calcium, magnesium, evaporating the solution to precipitate the alkaliferrpus iron,- cupric copper, lead, strontium, metal or ammonium salt; barium, manganese, zinc, cadmium, cobalt, and '5. A process of separating an inorganic salt the like. When these metals or their salts are, of the group consisting of sodium chloride, tri- 10 present as impurities, they may be held in solusodium phosphate and ammonium sulphate from tion durin Pre ip ta on of the ga c t y a solution containing the same and in addition adding suitable amounts of an alkali-metal an impurity of the group consisting of alkaline hexametaphosphatetothe solution. earth metal salts and salts of divalent heavy 115 I have deslcribed Several applications of my metals, which comprises adding an alkali-metal l5 invention. It is to'be understood, however, that hexametaphosphate to the solution to hold the these applications are illustrative only and that impurities in solution, and'evaporating the soluthe invention is not limited thereto, but maybe tion to precipitate the inorganic salt. otherwise embodied within the scope of the fol- 6.-A process of separating an alkali-metal or lowing claims. ammonium salt from a solution containing the 20 I claim: same and in addition an impurity of the group l. A process of separating an alkali-metal or consisting of alkaline earth metal 'salts and salts ammonium salt from a solution containing the of divalent heavy metals, which comprises add.- same and in addition an impurity of the group ing an alkali-metal hexametaphosphate to the consisting of alkaline earth metal salts and salts solution to hold the impurities in solution, and 25 of the divalent heavy metals, which comprises evaporating the'solution at a temperature below 'adding an alkali-metal hexametaphosphate to the about 100 C. to precipitate the alkali-metal or solution to hold the impurities in solution, and ammonium salt. evaporating the solution under vacuum to pre- 7. A process of separating an alkali-metal or cipitate the alkali-metal or ammonium salt. ammonium salt from a solution containing the 30 JD of separating an alkali-metal o same and in addition an impurity oi the group ammonium salt from a solution containing the consisting of alkaline earthmetal salts and salts same and in addition an alkaline earth metal salt of divalent heavy metals, which comprises adding as an impurity, which comp es adding an an alkali-metal hexametaphosphate to the solualkali-metal hexametaphosphate to the solution tion to hold the impurities in solution, and 35 to hold the alkaline earth metal salt in solution, evaporating the lution at a temperature below and evaporating the solution to precipitate the about 100 C. and under reduced pressure to alkali-metal or ammonium salt. precipitate the alkali-metal or'ammonium salt.

. 3. A process of separating an alkali-metal or 8. A process of separating an alkali-metal 40 ammonium salt from a solution containing the chloride from a solution containing the same and 40 same and in addition calcium salt as an impurity. in addition a calcium salt as an impurity, which which comprises adding an' alkali-metal hexacomprises adding an alkali-metal hexametaphosmetaphosphate to the solution to hold the calcium phate to the solution to hold the calcium salt in salt in solution, and evaporating the solution solution, and evaporating the solution to preto precipitate the alkali-metal or ammonium incipitate' the alkali-metal chloride.

I, v appears in the printed specification of the above numbered patent requiring correction as follows Page 2, first column, line 1 .5-1 .6; c1aim,- strike out the word "inorganid'g andth at the said Letters Patent should be read with this correction therein that the same may conform to the record. of the case in the Patent Office.- Q

Signed and sealed this 29th day of March, A. D. 1958.

Henry Arscl aie,

( l) Acting Commissioner'of Patents.

Patent No. 2,168,783.

lithium or ammonium hexametaphosphates.

The invention is applicable to the separation of inorganic salts from solutions containing as imorganic salt.

4. Aprocess of separating an alkali-metal or CERTIFICATE It is hereby certified that. error may be used; such, for example, as potassium,

ammonium salt from a solution containing the same and in addition an impurity oi the group consisting of alkaline earth metal salts and salts of divalentheavy metals, which comprises add- ,GEORGE w. sm'rn.

F CORRECTION I r y 5', 195a. GEORGE'W. s im.

purities any of the alkaline earth or divalent ing an alkali-metal hexametaphosphate to the heavy metals. For example, sodium hexa'metasolution to hold the impurities in solution, and phosphate holds in solution calcium, magnesium, evaporating the solution to precipitate the alkaliferrpus iron,- cupric copper, lead, strontium, metal or ammonium salt; barium, manganese, zinc, cadmium, cobalt, and '5. A process of separating an inorganic salt the like. When these metals or their salts are, of the group consisting of sodium chloride, tri- 10 present as impurities, they may be held in solusodium phosphate and ammonium sulphate from tion durin Pre ip ta on of the ga c t y a solution containing the same and in addition adding suitable amounts of an alkali-metal an impurity of the group consisting of alkaline hexametaphosphatetothe solution. earth metal salts and salts of divalent heavy 115 I have deslcribed Several applications of my metals, which comprises adding an alkali-metal l5 invention. It is to'be understood, however, that hexametaphosphate to the solution to hold the these applications are illustrative only and that impurities in solution, and'evaporating the soluthe invention is not limited thereto, but maybe tion to precipitate the inorganic salt. otherwise embodied within the scope of the fol- 6.-A process of separating an alkali-metal or lowing claims. ammonium salt from a solution containing the 20 I claim: same and in addition an impurity of the group l. A process of separating an alkali-metal or consisting of alkaline earth metal 'salts and salts ammonium salt from a solution containing the of divalent heavy metals, which comprises add.- same and in addition an impurity of the group ing an alkali-metal hexametaphosphate to the consisting of alkaline earth metal salts and salts solution to hold the impurities in solution, and 25 of the divalent heavy metals, which comprises evaporating the'solution at a temperature below 'adding an alkali-metal hexametaphosphate to the about 100 C. to precipitate the alkali-metal or solution to hold the impurities in solution, and ammonium salt. evaporating the solution under vacuum to pre- 7. A process of separating an alkali-metal or cipitate the alkali-metal or ammonium salt. ammonium salt from a solution containing the 30 JD of separating an alkali-metal o same and in addition an impurity oi the group ammonium salt from a solution containing the consisting of alkaline earthmetal salts and salts same and in addition an alkaline earth metal salt of divalent heavy metals, which comprises adding as an impurity, which comp es adding an an alkali-metal hexametaphosphate to the solualkali-metal hexametaphosphate to the solution tion to hold the impurities in solution, and 35 to hold the alkaline earth metal salt in solution, evaporating the lution at a temperature below and evaporating the solution to precipitate the about 100 C. and under reduced pressure to alkali-metal or ammonium salt. precipitate the alkali-metal or'ammonium salt.

. 3. A process of separating an alkali-metal or 8. A process of separating an alkali-metal 40 ammonium salt from a solution containing the chloride from a solution containing the same and 40 same and in addition calcium salt as an impurity. in addition a calcium salt as an impurity, which which comprises adding an' alkali-metal hexacomprises adding an alkali-metal hexametaphosmetaphosphate to the solution to hold the calcium phate to the solution to hold the calcium salt in salt in solution, and evaporating the solution solution, and evaporating the solution to preto precipitate the alkali-metal or ammonium incipitate' the alkali-metal chloride.

I, v appears in the printed specification of the above numbered patent requiring correction as follows Page 2, first column, line 1 .5-1 .6; c1aim,- strike out the word "inorganid'g andth at the said Letters Patent should be read with this correction therein that the same may conform to the record. of the case in the Patent Office.- Q

Signed and sealed this 29th day of March, A. D. 1958.

Henry Arscl aie,

( l) Acting Commissioner'of Patents. 

